QImage without using scaled() makes program crash when rendering on QML - c++

Question

I want to render images from the webcam onto QML view. And I have to transfer the format from OpenCV to QImage. Then I implement a QQuickPaintedItem singleton class to render QImage.
If I don't use QImage::scaled() in my code when I start grabbing and invoking rendering, my program crashes and I don't know why.
image=cv::Mat(stImageInfo.nHeight,stImageInfo.nWidth,CV_8UC3,m_pBufForSaveImage);
//cv::Size dsize = cv::Size(round(0.33 * stImageInfo.nWidth), round(0.27 * stImageInfo.nHeight));
//cv::Mat shrink;
//resize(image, shrink, dsize, 0, 0, CV_INTER_AREA);
QImage Qimag = MatImageToQt(image);
Qimag = Qimag.scaled(image.cols*0.33,image.rows*0.27,Qt::IgnoreAspectRatio,Qt::SmoothTransformation);
MyImage *myimg = MyImage::instance();
myimg->setM_Image(Qimag);
//render QImage to QML
#include "myimage.h"
MyImage::MyImage(QQuickPaintedItem *parent)
{
Q_UNUSED(parent)
}
MyImage* MyImage::myImage = new MyImage;
MyImage *MyImage::instance()
{
return myImage;
}
void MyImage::paint(QPainter *painter)
{
QRectF target(0.0, 0.0, 800.0, 550.0);//width*0.33 height*0.27
QRectF source(0.0, 0.0, 800.0, 550.0);
painter->setRenderHint(QPainter::Antialiasing, true);
painter->drawImage(target, this->m_Image, source);
}
const QImage &MyImage::getM_Image() const
{
return m_Image;
}
void MyImage::setM_Image(const QImage &mimage)
{
if (mimage != m_Image) {
m_Image = mimage;
emit m_ImageChanged();
}
}